1. Field of the Invention
The present invention relates to a plasma processing apparatus and, more specifically, a plasma processing apparatus for processing a base using a plasma as an excitation source, which serves as a plasma CVD apparatus capable of suitably forming a crystalline or nonmonocrystalline functional deposition film available for a semiconductor device, an electrophotographic photosensitive device, an image input line sensor, an image pickup device, a photovoltaic device, and the like, a sputtering apparatus capable of suitably forming an insulating film, a metal wiring film, and the like for a semiconductor device or an optical element, and an etching apparatus for a semiconductor device, and in which an RF wave of 20 MHz to 450 MHz can be suitably used.
2. Related Background Art
Various types of plasma processing apparatuses are used for semiconductors and the like in accordance with different application purposes. For example, a variety of apparatuses and methods have been used to enhance their features in, e.g., film formation of an oxide film, a nitride film, and an amorphous silicon-based semiconductor film using a plasma CVD apparatus and method, formation of a metal wiring layer using a sputtering apparatus and method, and micropatterning techniques using an etching apparatus and method.
Strong demand has recently arisen for improving the film quality and processing ability in a plasma processing method, and various implementations therefor have been examined.
Particularly, a plasma process with an RF power is used because it has advantages in stable electric discharge and thus formation of an insulating material such as an oxide film or a nitride film.
The oscillation frequency of a discharge RF power supply used for a conventional plasma process such as a plasma CVD process is generally 13.56 MHz. FIG. 1 is a schematic view for explaining the arrangement of a general plasma CVD apparatus frequently used in deposition film formation. The plasma CVD apparatus shown in FIG. 1 is an example of a film forming apparatus used when an amorphous silicon film (to be referred to as an a-Si film hereinafter) is to be formed on a cylindrical electrophotographic photosensitive body. A method of forming an a-Si film using this apparatus will be exemplified below.
In a reaction vessel 301 capable of pressure reduction, a cylindrical cathode electrode 302 electrically insulated from the reaction vessel 301 by an insulating material 311, and a cylindrical target film formation base (electrophotographic photosensitive base) 303 as a counter electrode are arranged. In order to improve uniformity in film thickness and film characteristics, the size of the cathode electrode 302 in the axial direction of a cylinder is larger than that of the target film formation base in the axial direction of the cylinder. For example, in order to reduce unevenness of a film thickness and obtain practically available uniformity in film thickness, the size of a cathode electrode in the axial direction of the cylinder must be at least about 1.5 to 2 times that of a target film formation base in the axial direction of the cylinder. Therefore, in the plasma CVD apparatus shown in FIG. 1, a relationship between the lengths of the target film formation base 303 and the cathode electrode 302 is kept so as to establish the above relationship (note that FIG. 1 does not accurately show this relationship). The target film formation base 303 is held by a rotation mechanism 304 driven by a motor M, and its interior is heated by an internal heater 305. An earth shield 306 is arranged around the cathode electrode 302 so as not to cause electric discharge between the cathode electrode 302 and the reaction vessel 301. An RF power supply 307 is connected to the cathode electrode 302 via a matching circuit 308. An evacuation means 309 and a gas supply means 310 are also provided.
After the interior of the reaction vessel 301 is evacuated to a predetermined vacuum degree (e.g., 1 mTorr or less) by the evacuation means 309, a source gas such as a silane gas, a disilane gas, a methane gas, or an ethane gas, or a doping gas such as a diborane gas is introduced by the gas supply means 310 to keep the interior of the reaction vessel 301 at several 10 mTorr to several Torr.
An RF power of 13.56 MHz is applied from the RF power supply 307 to the cathode electrode 302 to generate a plasma between the cathode electrode 302 and the target film formation base 303. The source gas is decomposed to deposit an a-Si film on the target film formation base 303 heated to about 100.degree. C. to 400.degree. C. by the heater 305.
Assume that a deposition speed for obtaining an a-Si film enough to satisfy to the performance of an electrophotographic photosensitive body according to this film forming method is set to form the a-Si film at a deposition speed of, e.g., about 6 .mu.m/h. If the deposition speed is further increased, no satisfactory characteristics of the electrophotographic photosensitive body may be obtained. When an a-Si film is utilized as a general electrophotographic photosensitive body, a film thickness of at least 20 to 30 .mu.m is required to obtain a satisfactory charging ability. For this reason, it takes a long period of time to manufacture an electrophotographic photosensitive body.
In recent years, a plasma CVD method using an RF power supply of 13.56 MHz or more in a parallel-plate plasma CVD apparatus is reported (Plasma Chemistry and Plasma Processing, Vol. 7, No. 3, (1987) PP. 267-273). This report suggests that the deposition speed can be increased by increasing a discharge frequency to more than 13.56 MHz without degrading the performance of a deposition film, which has received a great deal of attention. A sputtering method and the like using an increased discharge frequency are also reported. Recently, superiority of the increase in discharge frequency over other factors has been widely examined.
At the conventional discharge frequency of 13.56 MHz, the inductance of a cathode electrode can be ignored. However, when the discharge frequency is set at 20 MHz to 450 MHz, the inductance cannot be ignored. A plasma reaction is localized due to this inductance, so that it is difficult to uniform the discharge strength near a target film formation base on the entire target film formation base.
Such instability and nonuniformity in discharge strength cause nonuniformity in film thickness and film quality. In a film having structure-sensitive semiconductor properties, such as amorphous silicon used as an electrophotographic photosensitive body, optical and physical changes extremely sensitively appear on the film. For this reason, image degradation such as an image defect and unevenness of the density may be caused on a copied image.
When an electrophotographic photosensitive body as of amorphous silicon iS manufactured at a discharge frequency of 20 MHz to 450 MHz, the deposition speed is increased in comparison with a case using a discharge frequency of 13.56 MHz, but no great improvement in deposition speed has been attained. There is much room left unsolved for efficiency of productivity of products and a decrease in cost.